Development of secondary batteries has been ongoing in recent years due to rapid spread of digital cameras or smart phones or portable PCs, rising fuel price or increasing consciousness towards environmental burdens, and further, expectations for application to automotive power or electric storage for smart grids.
Used as electrodes for secondary batteries are a positive electrode having a positive electrode material comprising lithium ions and a conductive additive respectively adhered to the surface of a metal foil, and a negative electrode having a negative electrode material that allows adsorption/desorption of lithium ions and a conductive additive respectively adhered to the surface of a metal foil. This lithium battery has advantages such as high working voltage, high energy density, light weight, and long life expectancy, and active development has been continued as the best selection.
A composite having a metal compound that can absorb and desorb lithium supported on a carbon material that is employed as a conductive additive is often employed as an electrode material comprising lithium ions. Examples of a metal compound include lithium cobaltate, lithium iron phosphate, lithium titanate, and lithium manganese phosphate.
Here, development of electric vehicles (EV) or hybrid electric vehicles (HEV) where a part of its drive is assisted with an electric motor has been radically promoted at each automobile manufacturer, and a secondary battery having large capacitance and high input and output is desired as the power source thereof in these automotive applications.
In order to achieve large capacitance and high input and output when employing as the electrode, a composite having a metal compound that can absorb and desorb lithium supported on a carbon material that is employed as a conductive additive, nanosizing of the metal compound and the carbon material that configure the composite can be pointed out as one means therefor.
Accordingly, various methods for nanosizing a metal compound and a carbon material are currently being proposed. A representative method has a problem that in the manufacturing steps of a composite, even if nanosizing of a metal compound and a carbon material is successful during manufacture, aggregation of the metal compound and the carbon material occurs before completing composition of the metal compound and the carbon material, and thus secondary particles of the metal compound and the carbon material will finally be large.
In order to solve this problem, a technology targeting to simultaneously cause the nanosizing of a carbon material, the production of a metal compound precursor excluding lithium, the nanosizing of the metal compound precursor, and the adsorption of the metal compound precursor to the carbon material has been proposed (see e.g. Patent Document 1).
In this technology, each source material of the carbon material and the metal compound precursor are mixed in a container, the carbon material is dispersed by utilizing shear stress and centrifugal force by the rotation of the container, and at the same time, the metal compound precursor is produced by mechanochemical reaction, thereby carrying out dispersion of the produced metal compound precursor while adsorbing the metal compound precursor to the carbon material by mechanochemical reaction.
Moreover, functional groups such as carboxyl or hydroxyl groups present at the surface of the carbon material are electrochemically unstable, and thus the presence of such functional group to which a metal compound is not adsorbed may reduce the charge and discharge rates of the secondary battery and inhibit high input and output of the secondary battery. Accordingly, measures for decreasing the number of functional groups to which a metal compound is not adsorbed are taken, for example, coating is made on the carbon material or the functional group is chemically modified (see e.g. Patent Document 2).